1. Technical Field
The present disclosure relates generally to material handling systems, and is more particularly directed to gapping methods and systems that provide a desired gap between articles (e.g., cartons) within a continuously flowing train of articles.
2. Description of the Related Art
Establishing a correct gap between cartons within a continuously flowing train of cartons has become an increasingly difficult problem to solve over the last several decades. At low speeds, there is no real challenge. For example, if two cartons, traveling at 100 feet per minute (FPM) have only 1 inch of gap between them, but require 12 inches, the control system can easily create the required gap, because (at 0.5 g) the second carton can be stopped within an inch of travel after the previous carton has exited the conveyor. However, as the speed of the cartons increases, the conveyor takes longer to stop (at the same deceleration rate). Also, the carton is traveling at a higher average speed during that extended time. Hence, the distance required to stop a carton increases with the square of the increase in speed.
As a result, at 600 FPM, the travel distance which the second carton requires in order to increase the gap between itself and the previous carton (constantly moving at 600 FPM) by 11 inches is 46 inches. Obviously, there is not nearly enough space available to effect this change after the first carton exits the controlling conveyor.
Conventional single-stage gapping units must either give up or release at a speed which will usually cause the required gap to be “pulled” between the cartons. This method, while it can be reasonably effective with exotic algorithms applied, is based on assumed weight transfer points for cartons and inadvertently effects gapping of neighboring cartons. The result, unavoidably, is a level of ineffectiveness that worsens at higher speeds.
FIG. 1 illustrates a generally-known material handling system 100 that changes a gap between cartons 101, 102 utilizes a series of variable speed conveyors 104 of about three to four feet long. This initial gap is read on a fixed speed infeed conveyor 106 by a sensor such as a photo eye sensor 108. Speed and location of the cartons 101, 102 is tracked initially on the infeed conveyor 106 by monitoring an output from a rotary encoder 110 in relation to when a leading edge or trailing edge of a carton 101, 102 passes the photo eye sensor 108. The lead article 101 in this instance is transported at a velocity VIN on the infeed conveyor 106. The lead article is transported at a constant velocity Vout subsequently on the series of variable speed conveyors 104 that discharge onto a transport conveyor 112. On each of the series of variable speed conveyors 104, the position of the cartons 101, 102 is sensed by a respective photo eye sensor 108. Each variable speed conveyors 104 also includes a rotary encoder 110. In addition the significant amount of equipment thought necessary to dynamically track each carton 101, 102, the amount of gapping adjustment achievable at each variable speed conveyors 104 is limited. For example, the initial gap is determined at time t0. At time tj, both cartons 101′, 102′ have moved onto a first variable speed conveyors 104. When the following carton 102 is detected, the material handling system 100 waits for a period of time until time tk to allow the lead carton 101″ to transition to a second variable speed conveyor 104. The first variable speed conveyor 104 can then change velocity to affect a gap of the following conveyor 102″ for a short period of time that the following conveyor 102″ remains on the first variable speed conveyor 104. Subsequent variable speed conveyor 104 sense the cartons 101, 102 again respectively and make their own small contributions to affecting the gapping. Eventually, at time tf cartons 101′″, 102′″ each the transport conveyor 112 at the target gap spacing.
The generally-known material handling system 100 is quite lengthy in addition to requiring dynamic sensing during control of each conveyor stage. Their systems detect a carton and its leading gap as a carton enters a varying speed conveyor, the algorithm must wait for the prior carton to exit before it can affect a change to the gap, otherwise if both cartons were on the same conveyor, they would both move at the same speed and the gap would not change. Thus, the varying speed conveyor must be longer than the average carton plus the gap, and for best performance, it must be longer than the longest carton. Our system is a plurality of short belts—in our test system they are 6-inches long.